1. Field of the Invention
The invention relates to the production of ultrahigh purity silane and silicon. More particularly, it relates to an improved process for ensuring the removal of impurities, e.g., metal hydrides, to levels required for such production.
2. Description of the Prior Art
The development of new techniques for the utilization of non-polluting sources of energy is of paramount national and world-wide interest. Solar energy is among the energy sources of greatest interest because of its non-polluting nature and its abundant, non-diminishing availability. One approach to the utilization of solar energy involves the conversion of solar energy into electricity by means of the photovoltaic effect upon the absorption of sunlight by solar cells.
Silicon solar cells, the most commonly employed devices based on the photovoltaic effect, have been employed reliably in space craft applications for many years. For such applications and for industrial and commercial applications in general, crystals of ultrahigh purity, semiconductor grade silicon are commonly employed. Such high purity, high perfection silicon is generally prepared by procedures involving converting metallurgical grade silicon to trichlorosilane, which is then reduced to produce polycrystalline, semiconductor grade silicon from which single crystals can be grown. The costs associated with the production of such high purity, high perfection crystals are high.
The economic feasibility of utilizing solar cell technology for significant portions of the present and prospective needs for replenishable, non-polluting energy sources would be enhanced if the overall cost of producing single crystal wafers of requisite purity could be reduced. A major area of interest, in this regard, relates to the development of a low-cost, continuous process for the production of high purity polycrystalline silicon from metallurgical grade silicon. The need for such low-cost, high purity silicon is increased by the continued expansion of the utilization of solid-state electronic devices. While the purity requirements for solar grade silicon are not as stringent as for semi-conductor or electronic applications, the highest purity silicon material available at economically feasible costs can be effectively utilized for either solar cell or electronic applications.
The initial step of converting metallurgical silicon to trichlorosilane has commonly been carried out by reacting metallurgical grade silicon with HCl in a fluid bed reaction zone at about 300.degree. C. Trichlorosilane comprises about 85% of the resulting reaction mixture, which also contains silicon tetrachloride, dichlorosilane, polysilanes and metal halides. While this technique has been employed successfully in commercial practice, it requires the use of relatively large reaction vessels and the consumption of excess quantities of metallurgical silicon. In addition, the reaction mixture is relatively difficult to handle and has associated waste disposal problems that contribute to the cost of the overall operation.
In producing high purity polycrystalline silicon from trichlorosilane, current commercial technology is a low volume, batch operation generally referred to as the Siemens process. This technology is carried out in the controlled atmosphere of a quartz bell jar reactor that contains silicon rods electrically heated to about 1100.degree. C. The chlorosilane, in concentrations of less than 10% in hydrogen, is fed to the reactor under conditions of gas flow rate, composition, silicon rod temperature and bell jar temperature adjusted so as to promote the heterogeneous decomposition of the chlorosilane on the substrate rod surfaces. A general description of the Siemens type process can be found in the Dietze et al. patent, U.S. Pat. No. 3,979,490.
Polycrystalline semiconductor grade silicon made from metallurgical grade silicon costing about $0.50/lb. will, as a result of the cost of such processing, presently cost on the order of about $30/lb. and up. In growing a single crystal from this semiconductor grade material, the ends of the single crystal ingot are cut off, and the ingot is sawed, etched and polished to produce polished wafers for solar cell application, with mechanical breakage and electronic imperfection reducing the amount of useable material obtained. As a result, less than 20% of the original polycrystalline, semiconductor grade silicon will generally be recoverable in the form of useable wafers of single crystal material. The overall cost of such useable material is, accordingly, presently on the order of about $300/lb. and up. Because of the relatively large area requirements involved in solar cell applications, such material costs are a significant factor in the overall economics of such applications.
Development efforts to improve high purity silicon technology involve all aspects of the conversion of low-cost metallurgical grade silicon to the presently high cost, high purity silicon product, particularly to means for achieving requisite purity at an economically attractive cost. One aspect of such development work is directed to a process for the production of high purity silane from which silicon can be produced on a continuous basis as hereinafter described. This silane production process involves the hydrogenation of metallurgical grade silicon with hydrogen and silicon tetrachloride to form a gas stream containing trichlorosilane and dichlorosilane. The chlorosilanes are subjected to disproportionation in the presence of an ion exchange resin to progressively replace chlorine molecules with hydrogen molecules so that product high purity silane is eventually recovered from the process and silicon tetrachloride formed during the disproportionation reactions is recycled for further hydrogenation. The process is an integrated one utilizing hydrogen and metallurgical silicon as essentially the only consumed feed materials. The initial tri- and dichlorosilane producton step is preferably carried out at particular elevated pressures and temperatures, substantially enhancing the operation, conversion rate and the production rate obtainable in a given size reaction vessel. Unreacted silicon tetrachloride is conveniently recycled for reaction with additional quantities of hydrogen and metallurgical silicon. Waste disposal is readily accomplished, with material wastage minimized, by condensing a minor portion of unreacted silicon tetrachloride from the trichlorosilane reaction mixture, with said condensed silicon tetrachloride and accompanying impurities being passed to waste without the necessity for dilution prior to hydrolysis previously required during waste disposal.
The high purity silane thus produced can be further purified to remove residual impurities such as trace amounts of monochlorosilane, as required, and may be decomposed on a continuous or semicontinuous basis to produce high purity silicon, e.g., in a fluid reaction zone containing fluidized silicon seed particles or in the hot free space reaction zone of a decomposition reactor. By-product hydrogen produced in the silane decomposition operation can conveniently be recycled to the initial trichlorosilane production step and/or recycled for use as a carrier gas for the silane being decomposed or as a fluidizing gas in the fluid bed silane decomposition operation.
In such improved processing presently under development, the disproportionation reaction zone actually comprises a separation-disproportionation reaction zone in which progressive conversion of the higher chlorosilanes to lower chlorosilanes and to silane occurs, with staged separation until silane is removed as product and silicon tetrachloride is recycled back to the hydrogenation section. The conversion of the tri- and dichlorosilane feed stream from the hydrogenation reaction is accomplished in a combination of distillation columns and redistribution or disproportionation reaction zones. A process arrangement for this purpose was described in the Fourth Quarter, 1977 quarterly Progress Report submitted under the contract referred to above at the beginning of the specification. As described therein, a series of distillation columns is combined with disproportionation reaction zones that process the bottom fraction removed from each distillation column. The high purity silane product is taken from the top of the last distillation column in series, while recycle silicon tetrachloride is taken from the bottom of the first column in series. This process embodiment essentially separates the feedstock into a first trichlorosilane stream, with lighter components as the top draw from the first distillation column, then a dichlorosilane stream, with higher components, as the overhead draw from the second column, then monochlorosilane, and lighter components, as the top draw from the third column, and finally the silane product as the top draw from the last column. The bottom draw from each column is processed through a redistribution or disproportionation reaction zone in order to shift the feed chlorsilane fraction to a range of chlorosilane components as based on chemical equilibrium. This shift or equilibrium redistribution of the chlorosilane feedstock is commonly referred to as disproportionation as this term is used herein. The process stream from each disproportionation reaction zone becomes feedstock for the next preceding distillation column. On an overall basis, this arrangement has the function of processing the initial feedstock comprising primarily trichlorosilane, with minor amounts of lighter chlorosilanes, i.e., dichlorosilane, and silicon tetrachloride such that high purity silane is ultimately removed as product and silicon tetrachloride is recycled for further hydrogenation. Variations of this basic arrangement include the replacement of the final silane purification distillation column with a combined partial condensation and adsorption purification step. The partial condensation produces a high silane content vapor fraction and a liquid fraction containing primarily the lighter chlorosilanes. The liquid fraction can be processed in a redistribution reactor for recycle, whereas the vapor fraction is further purified in the adsorption step. Such purification step could conveniently operate carbon beds on a thermal swing cycle to produce an ultrahigh purity silane product and a bottoms stream for recycle.
The improved process as above described conveniently enables each lower boiling fraction to be separated from the mixture and passed to the following column, with the bottoms of each column being processed through a disproportionation reaction zone for recycle to the previous column. While such a process arrangement is very workable, it nevertheless requires essentially a paired column and disproportionation reactor for each chlorosilane component. Simplification and reduction of necessary processing equipment is always desired as part of the overall effort to reduce the cost of silicon and enhance the technical and economic feasibility of utilizing low-cost, high purity silicon for practical commercial solar cell and electronic applications.
The improved process arrangement also has a potential disadvantage related to the entrapment of impurities in recycle loops between columns, e.g., impurities at a relative volatility between dichlorosilane and silane such as the diborane (B.sub.2 H.sub.6) or phosphorous hydride (PH.sub.3) and impurities at an intermediate volatility between trichlorosilane and dichlorosilane such as boron chloride (BCl.sub.3).
The above-noted type of impurities present in the trichlorsilane-containing gas stream passed from the initial metallurgical silicon hydrogenation step, and not removed in the preliminary silicon tetrachloride condensation step referred to above, can generally be removed upon contact with the ion exchange resin employed in the disproportionation zone. However, since any chemical reaction or separation process can never be absolute and since the separation process involves recycle loops, it is a real possibility that a small impurity concentration could build up over an extended period of operating time. Such a buildup would then either impact on the silane product specification or overload the separation column system.
It is an object of the present invention to provide an improved silane process as part of a high grade silicon production process.
It is another object of the present invention to provide a column impurity control feature for the silane process to prevent trapping and buildup of intermediate volatility impurities within process recycle streams between adjacent columns.
It is another object of the present invention to provide bleed streams from the final two separation columns in the redistribution reactor and separation column section of the ultrahigh purity silicon production process to remove impurities that may build up within those process recycle loops.
It is another object of the present invention to remove slip streams from the bottom of the two separation columns and add those streams to the recycle silicon tetrachloride process streams in order to subject contained impurities to the waste removal reactions within the hydrogenation section of the silane process.
It is another object to remove slip streams from the two columns in the separation train in such a manner that the operation of those columns or other parts of the system are not subjected to upset or overload conditions.